Dental caries and periodontitis are the two most common infectious diseases in humans, but the risk for developing either disease is not uniform in the population. Identifying the determinants of susceptibility to disease is an important step in elucidating the etiology and pathogenesis of these diseases. Susceptibility to infectious diseases may be influenced by host-associated factors that affect either acquisition of bacteria or the host's response to bacterial challenge. The specific aim of this study is to determine to what extent genetic background is a factor in bacterial acquisition and colonization in the oral cavity. A twin model will be used, which can provide important information on the relative influences of host and environmental factors on disease susceptibility. Dental plaque samples have been collected from pairs of monozygotic and dizygotic twins, and will be analyzed using ribosomal operon DNA sequence analysis for identification of bacteria. Similarity of bacterial profiles from pairs of monozygotic and dizygotic twins will be compared. This approach will elucidate to what extent the host determines the composition of the bacterial community in the oral cavity. Relevance: Tooth decay and gum disease are very common diseases that are caused by bacteria in everyone's mouths, but only some people develop tooth decay or gum disease. It is not known if genetics plays a role in developing these diseases. It is important to understand all of the factors that cause tooth decay and gum disease, including genetic factors, so treatments can be improved. [unreadable] [unreadable] [unreadable]